Trimming and setting thrust

[asleep]

Peter GoldSmith wrote a great article on how to trim any (not just IMAC) plane from start to ends. But unless I'm very stupid, he never discussed Down - or up - Thrust.

www.mini-iac.com/Portals/57ad7180-c5e7-49f5-b282-c6475cdb7ee7/PGoldsmith%20on%20trimming.pdf

Personally, I run 0 degrees vertical thrust on a 3m Extra w/DA-150, slightly tail heavy so manouvers are 'crisp'.

Would love to know how other's tackle this one.


cheers
sleeper

[Shawn Berkheimer]

once the CG is set and the airplane will fly level at 1/2 to 3/4 trottle, try some full throttle up lines and see what the airplane does. I adjust up/down and right thrust to get as close of a hands off vertical as I can. As speed tapers off things will change so usually you will only get about 400-500 feet hands off.

Shawn

[Bruce]

For up/down thrust:
- trim the plane for straight & level flight at power
- make a horizontal pass at power and then chop the throttle
- without power, the plane will respond to the trim you put in to overcome any uneven up/down thrust. For example, if you have too much up thrust in the motor, you had to put in down trim to overcome it and achieve level flight. So, in this example, when you cut power, the plane will desend quickly telling you that you need to reduce the up thrust in the motor. If the thrust is close to correct, when you chop power the plane should continue level for a bit and then desend in a smooth arc that you would expect with loss of speed. Too much down thrust is simply the reverse of this example.
- Adjust by shimming the motor in the appropriate direction and try again.

This, like most trimming issues, is a compromise. What you are after is minimizing up/down variation with throttle changes for the style/speed/plane you fly.

[notorious_benny]

I trim the model so it tracks a perfect vertical downline and then turn things around. Heres a post I made a long time ago about the way I like to trim my model out, its not the way Goldsmith does it but it works for me and a few pilots that I fly IMAC with over here in Australia.

"Heres a trick I have learnt while flying IMAC. And this is the way I set up all of my models now. It not my method I pinched it from 3dubya, the trainer huck dude!

Set your cg to a conservative position by the manufacturers reccomendation. Also set thrust lines as suggested, we will play with them later though.

Now before taking off set your elevators to 'zero' trim ie the elevator is in line with the stab.

Now fly you model, you will most likely need to hold a little bit of up in to maintian level flight, this is normal. You need to fly a line where the cg has little bearing on the way the model behaves as well as the thrust lines.

So.... fly vertical downlines with the motor at slow idle. What we are looking for is to see if the model pitches either towards the canopy or the gear when no elevator input is given. This will tell you if your airframe is truly 'neutral' (incidence wise not cg)

My Yak 55sp passes this test with the elevators zero trimmed. If your model passes this test then it is time to check thrust lines (if it dosnt, for the time being you can give elevator trim until the model drops vertically down though really the incidence of the stabs/wing will need a tweak ouch!)..... So turn it around and fly vertical uplines to see what effect the motor has on the line. Ie up down/side thrust. Adjust until your model flies a true straight upline without the need for elevator input.


By performing the above we have our model as close to 'zero' trim as possible ie the wing, stab and motor are not 'fighting' each other. Now you can perform the 45 degree inverted line test that was outlined above by Kris W to fine tune your cg. I personally like the model to gently drop its nose in the 45 line so I adjust the cg for this trait. (Slightlt nose heavy)

The reason I outlined all the above is that if your thrust/incidences are all outta whack in the first place they will all effect the behaviour of the model when you roll inverted, especially inverted 45 lines. In all honestly I think that fine tuning the cg should be one of the last things you do.

Dont trim your model to maintain altitude straight and level, we arent flying circuits here are we. Keep it trimmed so that it will drop vertically perfectly straight. By doing this the model will behave very similarly inverted as if does upright. ie you will need about the same amount of down elevator while inverted as you do up elevator while upright, after all once your in advanced IMAC you spend just as much time inverted as upright. So why have to fight the model everytime its inverted.

As an aside a lot of models need more down elevator while inverted because they are trimmed to fly hands off while straight and level (ie normally a little bit of up elevator trimmed in)

Hope this helps. It has certainly helped me with my competition flying"

The thread in its original context can be found here............. Is it tailheavy?

This is just the way I find that works for me I am sure other people will chime in with theyre own theories. But I can say that this has worked on any model I have used it on, even my profile flew better after applying the techniques outline above.

At the moment my super extra is running quite a lot of upthrust which makes it fly IMAC fantastically well and took away most of the pitch coupling in knife edge flight as well.

[asleep]

Thank you all for your very clear feedback. If I get this right, I should go from novice to novice + 1 !!!!

cheers
sleeping

[Bob S]

Just after setting the CG, Peter talks about thrust angles.

[Dangerous Dan]

I usually follow peter's article!

[Tim G]

You should give benny's method a try, at least on the sim. The only thing you may have to re-learn is level up-right flight.

Tim

[notorious_benny]

	Quote: Originally Posted by asleep
	Thank you all for your very clear feedback. If I get this right, I should go from novice to novice + 1 !!!! cheers sleeping

I only just noticed you are from OZ! Where in Australia are you from, I am sure we will be able to hook you up with a local IMAC competitor to help you out.